[Nexus 5 Q&A, Help & Troubleshooting] akenis:
Well make up your mind! Before you said you wanted to use GV as your messaging app. Hence my advice before to pretty much ignore whatever you choose as default SMS app and just send/receive your text from within the GV app (Having selected "receive messages through GV app).

Now you're saying that you are trying to send messages from the default SMS app (hangouts). So what you are really saying is that you want to use a default SMS app (hangouts, or textra) and have the text go out your through your GV number? This is totally possible and what I am doing. Bear with me...

We have to understand the GV is NOT really SMS. It just intercepts SMS messages sent to your GV number and sends them to the GV app. These messages are coming to you phone through data, not the radio. Think of GV app operating in two different modes:

1) GV app can operate independently from your devices SMS (select "Via the GV app). You can send and receive messages from the GV app and they will use your GV number. None of these messages will be "forwarded" to the system app. In this scenario, you can also use the system app separately. Messages sent and received from the system default app wil not appear in the GV app, and will use your REAL phone number.

2) GV app can be used to relay these pseud SMS messages coming across your data to the default system app (select "via the messaging app"). In this case, messages sent to your GV number will appear in the GV app, and then be forwarded to the system app and use it's notification settings. Now this gets a little weird, but when this forwarding happens, the message received in the system app appears to come from an unknown phone number in some random area code. Not sure why... has something to do with the whole forwarding process. Luckily, if the real incomming number is in you contacts, then GV appends the contact name to the message. Now if you reply from within the GV app, it goes right back to the sender (no copy of your reply will appear in the system app, because you replied from within GV). If you reply from the system app, and that pseudo phone number... then it will forward back to the GV app (a copy will be there), translate back to the real phone number and send out the reply. And it will be from your GV number. The key is using that pseudo number that translates to the contacts real number. If YOU initiate a message from within the native app, then it just sends a regular SMS using your phones number, NOT your GV number (incidently, this is the only way to send MMS as of now, so you just have to tell the recepient to use the real phone number for pics). When I first figured this out, I used to just make sure that I always initiated the conversation from the GV app. The last neat trick is to save that pseudo number to the existing contact as either a pager (who uses those?) or a custom "pseudo GV" number. No you can initate the conversation from within the system app, and slect the pseudo number.... it will route to GV and appear to come from your GV number. If you use their real mobile number, it will appear to come from your actual phone number.

Clear as mud? Supposedly there are some changes coming to the way GV works. Lets hope they get this stuff a little more user friendly and allow SMS. Actually, I wouldn't mind just using the GV app if it didn't make multiple threads per contact. Good luck...